Ohmic heating apparatus using electrodes formed of closed microporosity material

ABSTRACT

An ohmic heating apparatus for heating flowable media which is flowing through a conduit comprises electrodes exposed to the flowing media and supply means connected to the electrodes and to the media to pass a heating current through the flowable media. The electrodes are made of a material of closed microporosity having a resistance to surface fouling. The material of closed microporosity is preferably made of vitreous carbon.

This invention relates to surface fouling resistant materials, and inparticular to such materials for use as structures which have to contactflowable media which deposit surface fouling deposits.

Ohmic heating is a well known technique for heating flowable media andis well established in the food processing industry where it meets theneed to cook or sterilise foodstuffs. The basic principle is to arrangefor an electric current to flow directly in the medium between a pair ofelectrodes. Such direct ohmic heating cf the medium can permit highrates of heat input to the medium thus enabling rapid heating. Proposalsfor ohmically heating liquids, specifically pasteurising milk, aredescribed in the article "Pasteurisation of Milk by Electricity" byMcDowall, pp 275-291 The New Zealand Journal of Science and Technology,February 1929. In the "Electro-Pasteur" described in this article,electric current flows between electrodes spaced at intervals along apipe through which milk is flowing. The current flow is thereforeessentially parallel to the flow of milk. Reference may also be made toGB-B-2067390 which describes a particularly advantageous form ofelectrode for use in ohmic heating apparatus. Platinum or platinumcoated electrodes are commonly used to provide corrosion resistance insuch devices

In addition to heating foodstuffs, there is also a requirement to heatflowable media such as corrosive chemical reagents up to a requiredreaction temperature. Hitherto, attempts to apply the technique ofohmically heating such reagents have met with difficulties associatedwith surface fouling of the electrodes, in particular the deposition ofcrystals from the reagants. With many reagants the crystal growth isitself electrically insulating so that continued deposition results in anon-functional electrode. Often the deposition process is sufficientlyrapid to render ohmic heating unworkable due to the frequency of havingto clean or replace the fouled electrodes.

In accordance with the present invention, surface fouling resistantmeans arranged to contact flowable media comprises material of closedmicroporosity.

It has been found experimentally that such materials are unexpectedlyresistant to surface crystal growth. Whilst the reason why materials ofclosed microporosity should exhibit this property is not fullyunderstood, it is presently believed that some other sorts of materialsurfaces are vulnerable to fouling because they allow for trapping ofpockets of reagants in the open structures or pores cf the materialsurface. Such pockets may be the site for nucleation and crystal growthNo such trapping of pockets of reagants may occur with materials ofclosed microporosity, thus rendering such materials resistant to surfacefouling.

Conveniently, ohmic heating apparatus may comprise electrodes made ofsuch closed microporosity material, as may other components in suchapparatus where fouling by crystal deposition is detrimental.Preferably, the closed microporosity material is vitreous or glassycarbon. Conducting ceramics may also be suitable. It may be appreciatedthat although such materials are particularly suitable for structureswhich have to contact flowable media which rapidly deposit surfacefouling crystals on other sorts of electrodes, their applicability is infact to any sorts of flowable media where surface fouling might be aproblem It may also be appreciated that the material may also have to beelectrically conducting and chemically inert.

Another aspect of the invention is the use of closed microporositymaterials for surface fouling resistant means arranged to contactflowable media.

An example of the invention will now be described with reference to theaccompany drawing, the sale view of which illustrates an ohmic heatingapparatus embodying the present invention.

In this figure, there is indicated generally at 20 a pipe for theflowable medium which is to be heated. The medium is typically corrosivechemical reagants which have to be heated to attain a desired reactiontemperature. The pipe 20 is connected, in use, to means such as apositive displacement pump for producing a flow of the medium in thedirection of arrow 24 through the pipe. Such means might comprise a pumpconveying the reagants from one vat to another. As shown in the drawing,the pipe 20 has sections 21, 22 and 23 which are made so that thematerial flowing inside these sections is electrically insulated fromthe outside of the pipe. To achieve this the pipe sections may be madeentirely from insulating materials, or the pipe sections may beinternally lined with an insulating material. Typically, this materialmay be glass It is preferable that the insulating material employed is a"good" insulator, i.e. having a very low electrical conductivity,although the apparatus can be made to work provided the conductivity ofthe insulating material employed is less than that of the medium flowingin the pipe

The insulating pipe sections 21, 22 and 23 space apart four vitreouscarbon electrodes 25. The electrodes 25 are each arranged to haveelectrode surfaces which are exposed to medium flowing in the pipe 24. Athree-phase autotransformer 26 connected to a three-phase step uptransformer 27 is arranged to provide a variable alternating voltagesupply, for example from the mains three-phase electricity supply at 440V. The delta-connected secondary windings of the transformer 27 areconnected to the electrodes 25 with one terminal of the secondarywindings connected to earth and to the electrodes at either end of theheater portion of the pipe 20, i.e. the uppermost and lowermostelectrodes 25 in FIG. 1. The other two terminals of the secondarywinding are connected to respective ones of the two intermediateelectrodes 25. It can be seen, therefore, that a different phase ofalternating voltage is applied between each adjacent pair of electrodes25, but in each case the R.M.S. voltage applied across the electrodes isthe same. Having the outer two electrodes earthed minimises any risk cfcurrent flowing in the medium either before the inlet or after theoutlet of the heating portion of the pipe 20. Provision may be made tocool the interior surfaces of the pipe sections 21, 22 and 23 forexample by forming these pipe sections as double-skinned pipes with awater-cooling jacket between the skins. Cooling the pipe sections hasbeen found desirable to reduce fouling of the internal surfaces of thesepipe sections. Alternatively, the internal lining of the pipe sectionsmay themselves be of vitreous carbon, obviating the requirement forcooling.

I claim:
 1. In an ohmic heating apparatus for heating electricallyconductive flowable media flowing through a conduit, comprising at leastone electrode exposed to the flowing media and supply means electricallyconnected to said at least one electrode and to the media flowingthrough the conduit to pass a heating current though the electricallyconductive flowable media, the improvement comprising said at least oneelectrode made of closed microporosity material.
 2. The ohmic heatingapparatus of claim 1 wherein the electrode is made of vitreous carboncomprising the material of closed microporosity.